Forge welding includes the bonding together of metal parts, such as plates. For example FIG. 1(a) illustrates a typical partial T-joint forge weld 101 made between an edge of plate 103 and surface of plate 105, and FIG. 1(b) illustrates a typical square butt partial forge weld 107 made between facing edges of plates 109 and 111. Forge welding also includes the bonding together of edge portions of a folded metal sheet or strip wherein the edge portions are forced together at a weld point as the strip is longitudinally advanced in the direction of the formed weld seam. For example in FIG. 2, tube 113 is formed from a metal strip forced together at weld point 115 to form weld seam 117 as the strip advances in the direction of the single headed arrow and pressure force is applied in the directions indicated by the double headed arrows to force the edge portions of the strip together.
In a forge welding process high pressure is applied at the weld point, which is heated to the welding temperature, to produce the weld. Generally the welding temperature is below, but possibly near to, the melting point of the metal being weld. Heating the metal to welding temperature may be accomplished by using a suitable source of energy, such as a laser, electron beam, electric resistance or high frequency electric induction.
A forge welding process results in the creation of a heat affected zone (HAZ), which is the portion of the metal that was not melted during the welding process, but whose microstructure and mechanical properties were altered by the heat from the process. For example in FIG. 2 dotted lines 118 indicate the generalized outer boundary of the HAZ on either side of weld seam 117. As more clearly shown in FIG. 3(a) and FIG. 3(b) the width of the HAZ, XE, is equal to the distance between outer boundary lines 118. While in practice the outer boundaries of the HAZ may not be uniformly linear along the entire length of the weld, the width of the HAZ may be generally approximated by linear boundary lines. Minimizing the width of the HAZ generally minimizes the amount of metal that has properties different from those of the unheated metal part. The preferred or effective width of the HAZ is a complex function of many welding parameters including, but not limited to, the welding frequency, part wall thickness, part geometry, weld heating length, and angle and speed of the part at the weld point.
One particular application of induction forge welding is high frequency induction tube and pipe welding wherein high pressures are applied for very short periods of time, but right at the melt point temperature, to two edges of a strip forced into an oval shape by a tube forming machine before the adjacent edges of the strip reach the weld point as diagrammatically illustrated in FIG. 2, FIG. 3(a) and FIG. 3(b). At this temperature diffusion rates in the solid phase are very high and a quality bond results in a very short period of time. Ideally all of the melted metal should be squeezed from the bond plane into the inside or outside diameter weld beads, and the bond has no melted or cast metal. In FIG. 2 induction power can be supplied from a suitable ac power source (not shown in the figure) to induction coil 121 to induce current in the metal around a “V” shaped region formed by forcing edges of the strip together. The induced current flows around the back of the tube and then along the open “V” shaped edges to weld point 115 as illustrated by the typical flux line 119 (shown as dashed line) in FIG. 2. The length, y, of this “V” shaped region is approximately equal to the distance between the end of the coil closest to the weld point and the weld point. Generally since this length is relative to a particular forge welding machine, other definitions of this distance may be used as long as the defined distance is consistently use for a particular forge welding machine. The length, y, can also be referred to as the weld heating length. While a solenoidal coil is shown in FIG. 2 other coil arrangements may be used.
The effective width of the HAZ is a complex function of many welding parameters including, but not limited to, the welding frequency, component wall thickness, component geometry, weld heating length and angle, part joining speed, and part material. The following illustrates how these parameters can be mathematically applied.
The electrical reference depth, ξ, or penetration depth, which defines the distance from the edge of the metal part at which the induced current decreases approximately exponentially to e−1 (0.368) of its value at the surface, when the process is an induction forge welding process, can be calculated from equation (1):
  ξ  =            ρ              π        ⁢                                  ⁢        f        ⁢                                  ⁢        μ            
where ρ is the electrical resistivity of the metal part, μ is the relative magnetic permeability of the metal part, f is the electrical welding frequency of the supplied power, and π is the constant pi (3.14159).
The thermal reference depth, δ, or thermal diffusion depth, which represents how deeply the edge is heated by thermal conduction, may be calculated from equation (2):
  δ  =                    π        ⁢                                  ⁢        ɛ        ⁢                                  ⁢        y                    4        ⁢        v            
where ε is the thermal diffusivity of the metal part, y is the length of the “V,” which is also referred to as the weld heating length, and v is the speed at which the metal part passes the weld point, which is also referred to as the weld velocity.
There is a functional relationship between the electrical reference depth and width of the HAZ when both of these quantities are normalized by the thermal diffusion depth.
A normalized electrical reference depth, Zn, can be calculated from equation (3):
      Z    n    =            ξ      δ        .  
Normalized width of the HAZ, Xn, can be calculated from equation (4):Xn=a0+a1Zn+a2Zn2+a3Zn3.
Equation (4), or the normalized width of the HAZ polynomial, can be established by experimental forge welding of specific types of metal materials. For example each of empirical data points x1 through x18 in FIG. 4 represent a normalized electrical reference depth (Zn) and corresponding normalized width of the HAZ (Xn). Any suitable model can be used to fit the collected empirical data to a curve. In this particular example a suitable non-linear curve-fitting model is used to fit the data points to an equation with the polynomial form of equation (4) as diagrammatically illustrated by polynomial curve p1 in FIG. 4. The polynomial is generally of the form Xn=f(Zn) and the coefficients a0, a1, a2 and a3 in equation (4) represent coefficients derived for a specific material in the experiments or trials that resulted in the empirical data points.
Effective weld power, PE, can be calculated from equation (5):PE=H∘γ∘XE∘h∘v
where H is equal to the enthalpy of the forge welding process; that is, the change in enthalpy (measured in joules when PE is calculated in watts) of a metal in a forge welding process wherein the temperature of the metal is raised from its pre-weld temperature to its weld temperature;
γ is the density of the metal (measured in kilograms per cubic meters);
XE is the effective width of the heat affected zone (measured in meters);
h is the thickness of the metal being welded together (measured in meters); and
and v is the speed of the metal being welded at the weld point, or weld velocity (measured in meters per second).
One object of the present invention is to achieve a forge weld with a forge welding machine by specifying the preferred width of the heat affected zone for the weld and preferred weld temperature in the forge welding of one or more materials without knowledge of the required forge welding machine operating frequency or operating power setting.
Another object of the present invention is to set the operating frequency and operating power setting of a forge welding machine in a forge welding process to achieve a desired weld without input of the frequency and power settings by an operator of the forge welding machine.